You will not believe (or perhaps most of you seasoned Commodore equipment users will) how much time in troubleshooting costed me putting an old diskette from e-bay in my Commodore disk drives.
It all started when my GEOS-128 set arrived from e-bay. Two manuals, 6 diskettes, all visually in a very good shape. The smell of these items suggested they possibly spent long time at the bottom of some closet. No indication of dampness. The pages in one of the books were only yellowed a little bit. The diskettes looked clean (perhaps except one, which only you could notice by looking at the surface at certain angle that there is some residue on it, which of course I noticed after all my troubles).
So, I opened the GEOS manual on the page with instruction how to boot the system for the first time, I put the system diskette in my primary drive (I use three 1571 drives, first one with Jiffy DOS installed and a uIEC-SD device, this setup works very nice with CP/M) and booted it. The 1st disturbing thing to notice was a whining sound from the drive. The disk did not boot of course. I made a few extra attempts with the same result.
"OK" - I thought. - "This is just an old disk, I will try to read its directory."
This of course did not work, but perhaps because it is a GEOS diskette?
Anyway, I tried in my other drives. Then I put some of the other diskettes that arrived with the set in all of my drives and tried to read them. None has worked.
This is kind of expected and it happens with old diskettes. The most disturbing part however was that my normally working disks now has stopped reading too in any of my disk drives.
"What is it?" - I thought. - "The day of broken Commodore drives?".
I realized it is rather unlikely that all of the disk drives went down the same day. But, still clueless, I opened my first drive to see for obvious mechanical issues. Did not find any (even magnetic head looked clean). Next I opened my worst 1571 (the one I owned the longest time and looks like the one most worn down by time) and attempted to align it. Nothing worked. I gave up some time in the morning.
The next day I got my revelation moment - "A dirty head!".
Yep! That was it. These old disks must have been stored in some not too good conditions, perhaps for some time exposed to moisture and/or high temperature. Their magnetic layer literally peeled off their surface and it deposited on magnetic heads choking them up! I swabbed them with alcohol and they are all like new again.
I know most of the Commodore or any retro computer equipment users know this stuff. But if you are a young inexperienced retro computer technology enthusiast, beware! Better to have a designated disk drive (instead of your best primary disk drive) for reading/examining the diskettes from unknown sources, especially if they are listed as untested or their condition is not mentioned by the selling party. And if your equipment stops working as expected, first look for the obvious reasons (like neglected maintenance in my case, head cleaning was long overdue, regardless of the dirty diskettes put in recently). I swabbed the heads but I think it is better (and more convenient) to use one of those head cleaning diskettes, which unfortunately are not being made these days. I saw them on e-bay though and I am going to order some for myself.